The present invention relates to apparatus and/or process for controlling a gas sensor, such as a gas sensor including at least one cell which includes a solid electrolyte member and a pair of electrodes formed on the solid electrolyte member. Specifically, the present invention relates to a technique for sensing the impedance of a cell of the gas sensor, and controlling the supply of electricity to a heater of the gas sensor in accordance with the sensed cell impedance.
A gas sensor, such as oxygen sensor and air fuel ratio sensor, is widely used for an internal combustion engine for a motor vehicle or other applications, to improve the fuel consumption and control the combustion. Furthermore, in conjunction with the environmental concerns and tightening of regulations for limiting exhaust emissions of motor vehicles, the demand for reducing the amount of nitrogen oxides (NOx) in the exhaust gas mixture increases and drives the development of an NOx sensor capable of sensing the concentration of NOx directly. These gas sensors use a gas sensing element including one or more cells each including a solid electrolyte member of an oxygen ion conductive solid electrolyte such as zirconia, and a pair of electrodes formed on the solid electrolyte member. The gas sensing element produces an output from which the concentration of a specified gas can be determined.
As the gas sensor, there is known a full range air fuel ratio sensor (also called UEGO sensor) for sensing the oxygen concentration of a measurement gas with a construction which includes two cells (oxygen concentration sensing cell and oxygen pump cell) disposed on both sides of a measurement chamber to which a measurement gas is introduced through a diffusion resisting member. Moreover, there is known an NOx gas sensor including a cell for sensing the NOx concentration in addition to the two cells (oxygen concentration sensing cell and oxygen pump cell).
The gas sensor is connected with a sensor drive circuit for supplying electricity to a cell of the gas sensor, and arranged to sense the concentration of a specified gas in the measurement gas (gas mixture) from an output of the cell. Such a gas sensor includes a heater for heating the cell (the solid electrolyte member of the cell) to a higher temperature higher than or equal to an activation temperature so that the solid electrolyte member becomes oxygen-ion-conductive. The current supply to the heater is controlled to activate the cell quickly and to hold the cell active at temperatures equal to or higher than the activation temperature stably.
A Japanese patent document (JP 10-48180A) shows a temperature control system arranged to sense the impedance of a cell (element impedance or cell impedance) which varies in dependence on the temperature of the cell, and to control the temperature of the cell by controlling the current supply to the heater. It is possible to control the temperature of the cell by controlling the amount of electricity supplied to the heater so as to bring the impedance of the cell sensed periodically, to a desired target impedance. It is possible to sense the cell impedance by inputting a impedance sensing signal (or current) into a monitored cell whose impedance is to be sensed, and to determine the impedance from an output (response signal) of the monitored cell responsive to the impedance sensing signal.
If a wiring line leading to the monitored cell is disconnected by some trouble, the impedance of the monitored cell sensed by the control system increases excessively or infinitely. As a result, the control system misjudges that the cell (gas sensor) is cold, and continues supplying the maximum effective voltage to the heater. The continuation of application of the maximum effective voltage might increase the temperature of the gas sensor abnormally and damage the gas sensor. Therefore, a Japanese patent document (JP 2000-121600) proposes a diagnostic system for detecting an abnormal condition such as disconnection or short-circuit of gas sensing element and heater by monitoring an unusual variation of the element impedance.